Conventionally, in particular, as a valve suitable for a high-pressure fluid, an eccentric-type butterfly valve has been generally known. For example, in a double-eccentric-type butterfly valve, a valve shaft is provided to be eccentric to a flow path side with respect to a valve body, a virtual cone is provided from a predetermined point on a center line of a flow path toward the outer circumference of the valve body, and a contact surface on this cone with the valve body serves as a seal surface of a fluororesin-made seat ring. Even at the time of high pressure, sealability at the time of valve closing is enhanced more than a central-type butterfly valve. A seat ring for use in this eccentric-type butterfly valve is normally formed of fluororesin, which is a heat-resistant material, so as to be able to resist even a high-temperature fluid. Since fluororesin has a low elastic force compared with normal resin materials, fluororesin is formed into a special shape to improve the elastic force and enhance sealability with the valve body.
As an eccentric-type butterfly valve of this type, for example, a butterfly valve of PTL 1 has been disclosed. In this butterfly valve, a seat ring is made of synthetic rubber or fluororesin, and is fixed between a valve main body and a retainer with screws. On a retainer side of the seat ring, an inner end of the seat ring is in contact with a valve body for close contact, and a recessed part is formed to absorb a bulge at the time of distortion.
In an eccentric butterfly valve of PTL 2, in a schematic diagram of FIG. 11, a seat ring 1 is formed of an annular body in a flat plate shape. On one surface of this annular body, an annular groove 2 is provided by drilling concentrically with the annular ring. With this, the inner circumferential surface is formed in a convex shape in section, and a seal surface part 3 with respect to a valve body is provided. The annular groove 2 is provided so that its groove bottom 4 is present in an area E between a curved surface B formed by a line passing through a contact point A between the seal surface part 3 and the valve body and perpendicular to the outer circumferential surface of the valve body and a parallel plane C passing through the above-mentioned contact point A and parallel to a front surface. The seat ring 1 is divided into an outer half part 5 lower than a broken line in the drawing and an inner half part 6 upper than the broken line by taking the annular groove 2 as a boundary. The outer half part 5 fits in the recessed portion provided by drilling a valve main body, and is pressed by a retainer to be fixed to the valve main body. The inner half part 6 is given flexibility by the annular groove 2, and is provided so as to be sealable with the valve body.
PTL 3 suggests an eccentric-type butterfly valve in a structure in which a flowing direction is restricted to one direction and sealing is made in a direction in which operation torque increases with an increase in fluid pressure. In this eccentric-type butterfly valve, a groove part fixed by fitting in a protruding part provided to a retainer is provided on one surface side of an annular seat ring, and is provided as a flat shape so as to be able to catch both surfaces including this surface. In a state in which both of the entire flat surfaces abut on the retainer and a body from the groove part to a fixing part on an outer circumferential side, the seat ring is fixed as being caught by these. A seal surface of the seat ring is formed in a shape approximately identical to that of a spherical surface region on a seal side of a disc, and this seal surface has a seal width provided to have a dimension approximately equal to that of the sheet thickness of the seat ring. Thus, at the time of valve closing, sealing is made by a wide abutting surface between the seat ring and the disc.
In a seat ring of an eccentric-type butterfly valve of PTL 4, an annular pocket-shaped recessed part is concentrically formed, and a base part side of this recessed part is interposed between a main body and a retainer. A projection is formed on the retainer, and this projection is engaged in a base part to inhibit pullout of the retainer in an inner diameter direction and leakage of fluid from a back side of the seat ring to a secondary side. Also, to provide a gap between a movable part extending from a base part interposed between the main body and the retainer and the main body, the thickness of the movable part is formed so as to be smaller than the base part. And, the movable part is provided so that, when fluid pressure is applied to the pocket-shaped recessed part, the movable body is deformed in a whirling manner to be deformed so as to dig into a valve body.